You can register in person at City Hall or you can mail your registration form if it is postmarked tomorrow (the 15th). The Rock the Vote website has a simple web page that to create the form you can put in the mail.

You must re-register to vote if you have moved to Boston, changed your name, or would like to change your party enrollment. If you move within the City of Boston, you must update your registration by notifying the Boston Election Department of your change of address in writing.